This invention relates to a method of controlling a brake system provided with an automatic brake control device an an ABS controller
As an automatic vehicle operation device, a constant-speed travelling device has been put to practical use. In recent years, a car-to-car distance maintaining device has been developed in which the vehicle is accelerated and decelerated so as to follow a preceding vehicle. In vehicles carrying such an automatic operating device, besides a conventional brake device operated by the driver, an automatic brake control device is provided which activates automatic braking based on judgment of the automatic operating device.
On the other hand, ABS controllers for anti-skid control are mounted on many vehicles in recent years. Ordinary ABS controllers basically adopt independent control in which braking forces of the wheels are independently controlled according to the slip states of the respective wheels. But some ABS controllers allow to selectively use independent control and select-low control according to the degree of slip of the wheels and the yawing state of the vehicle. In the select-low control, the braking force of one of right, and left wheels that is smaller in slip is controlled so as to conform to the braking force of the other wheel that is greater in slip. Although the braking ability is lower than in the independent control, yawing of the vehicle can be suppressed more.
With a vehicle provided with both an automatic brake control device and an ABS controller, if a slip state of a wheel is detected while automatic braking is being done, ABS control is carried out in independent control. In such a case, if conditions of road surfaces the right and left wheels contact,differ from each other, the steering wheel may become uncontrollable due to the difference in the braking force between the right and left wheels, or the vehicle may yaw. At this time, since the driver, who is not operating the brakes, is not prepared for braking of the vehicle, there is a danger that he may make an error in operating the steering wheel, thus making the vehicle""s attitude unstable.
The following three types of measures have been proposed against such a danger in carrying out the ABS control while automatic braking is being done.
The first is to tackle on the automatic brake control device, such as stopping activation of automatic braking while ABS control is being carried out, or keeping constant the brake pressure for automatic braking (JP patent publication 6-1229).
The second is to alarm the driver, specifically judging the activation of automatic braking and the execution of ABS control by an AND circuit to activate an alarm device (JP utility model publication 5-19042).
The third is to tackle on the ABS controller, namely, when carrying out ABS control while automatic braking is being done, it is judged that it is traveling on a low-friction coefficient road where slip tends to occur, and the pressure reduction starting threshold for the wheel speed is set higher than in independent control to quicken the pressure reduction starting timing (JP patent publication 2001-138881). Specifically, there are proposed a method in which the pressure reduction starting threshold for the rear wheels, which has a larger influence on yawing, is set higher than that for the front wheels, and a method in which the pressure reduction starting thresholds of all the wheels in the first control cycle are set high.
In the first method, automatic braking is stopped or suppressed in a situation where deceleration is needed. Thus it accompanies danger of collision or abnormal approach.
The second method in which an alarm is given, may surprise the driver, so that he or she may make an error in operating the steering wheel.
In the third method by the ABS controller, one can expect an effect of suppressing out-of-control of the steering wheel and yawing by carrying out ABS control without surprising the driver while ensuring activation of automatic braking.
But road surfaces on which the vehicle actually travels differ widely. Since in the method described in JP patent publication 2001-138881, it is necessary to evenly set a pressure reduction starting threshold separately from normal independent control, assuming a low friction coefficient road beforehand without grasping the actual road surface conditions, it is difficult to set a proper pressure reduction starting threshold and the pressure reduction starting timing tends to be earlier than necessary under normal road surface conditions.
An object of this invention is to make it possible to carry out proper ABS control depending on the actual road surface conditions while automatic braking is being carried out.
According to this invention, there is provided a method of controlling a brake system including an automatic brake control device for activating automatic braking based on judgment by a constant-speed travelling device or a car-to-car distance maintaining device, and an ABS controller for controlling the braking forces of individual wheels based on slip states of the wheels, characterized in that if a slip state of the wheel is detected during activation of the automatic braking, control of braking forces by the ABS controller for both of the front and rear wheels is set at select-low control in which the braking force of one of the right and left wheels of which the slip is smaller is controlled in accord with the braking force of the wheel of which the slip is larger.
By setting the ABS control carried out during activation of automatic braking at the select-low control mode for both the front and rear wheels, it is possible to prevent yawing and prevent the steering wheel from getting out of control by applying braking forces such that slips of the right and left wheels produced due to actual road surface conditions will be equal to each other. In the select-low control mode, the braking force is not much different from the one in the independent control mode while there is no or only a small difference in slip of the right and left wheels, that is, while there is little possibility of the steering wheel getting out of control or yawing. Thus it is possible to ensure maximum braking force according to the actual road surface conditions.
If the driver operates the brake during the select-low control, control of the braking force by the ABS controller is set at the independent control in which the braking forces of the right and left wheels are independently controlled, at least for the front wheels. Thus, while the driver is prepared for braking of the vehicle, at least the front wheels, which contribute greatly to the braking force, are set at the independent control. This increases the braking capability.